US5602762A - Digital sample rate conversion - Google Patents

Digital sample rate conversion Download PDF

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Publication number
US5602762A
US5602762A US08/211,648 US21164894A US5602762A US 5602762 A US5602762 A US 5602762A US 21164894 A US21164894 A US 21164894A US 5602762 A US5602762 A US 5602762A
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Prior art keywords
filter
digital
sample rate
digital filter
signal
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US08/211,648
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English (en)
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David Lyon
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Snell Advanced Media Ltd
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Snell and Wilcox Ltd
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H17/00—Networks using digital techniques
    • H03H17/02—Frequency selective networks
    • H03H17/06—Non-recursive filters
    • H03H17/0621—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing
    • H03H17/0635—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing characterized by the ratio between the input-sampling and output-delivery frequencies
    • H03H17/065—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing characterized by the ratio between the input-sampling and output-delivery frequencies the ratio being integer
    • H03H17/0657—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing characterized by the ratio between the input-sampling and output-delivery frequencies the ratio being integer where the output-delivery frequency is higher than the input sampling frequency, i.e. interpolation
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H17/00—Networks using digital techniques
    • H03H17/02—Frequency selective networks
    • H03H17/06—Non-recursive filters
    • H03H17/0621—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing
    • H03H17/0635—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing characterized by the ratio between the input-sampling and output-delivery frequencies
    • H03H17/0685—Non-recursive filters with input-sampling frequency and output-delivery frequency which differ, e.g. extrapolation; Anti-aliasing characterized by the ratio between the input-sampling and output-delivery frequencies the ratio being rational
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N7/00—Television systems
    • H04N7/01—Conversion of standards, e.g. involving analogue television standards or digital television standards processed at pixel level
    • H04N7/0102—Conversion of standards, e.g. involving analogue television standards or digital television standards processed at pixel level involving the resampling of the incoming video signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N7/00—Television systems
    • H04N7/24—Systems for the transmission of television signals using pulse code modulation

Definitions

  • This invention relates to the sampling of video or other signals.
  • sampling frequency When converting video signals to digital form, there are conflicting criteria in selecting the sampling frequency. For historical reasons and because of certain regulatory standards, much existing hardware operates at arbitrary fixed frequencies. A commonly used frequency is 13.50 MHz. For other reasons, it is often preferred to use a sampling rate which is an integral multiple of the colour sub-carrier frequency. There are particular advantages in sampling at four times the sub-carrier frequency, these advantages including a simple demodulation procedure and a convenient phase relationship between pixels in neighbouring lines. On this theoretical basis, a sampling frequency of 17.73 MHz might for example, be selected for PAL.
  • the signal is converted to analogue, using of course the original sampling frequency; low pass filtered and--usually--amplified.
  • the analogue signal is then reconverted to digital at the desired sampling frequency.
  • the frequency response of the low pass filter is required to be of high quality and is generally specified in the regulatory standards such as CCIR 601.
  • the order or quality of frequency response of a digital filter can be related to the number of samples employed. It will be recognised that very close parallels exist in this context between digital filtering and the interpolation between digital signal values. In deriving an estimated value, away from a sample point, by interpolating from neighbouring sample points, accuracy is of course increased as the number of samples included within the interpolation, is increased. By the same token, the frequency response of a digital filter will improve as the number of samples (or in filter terminology, the number of taps) is increased. It can be estimated, for example, that to achieve the quality of frequency response laid down in CCIR 601, a digital filter would require to have at least 32 samples or taps.
  • a filter In deriving a number of sample values about a specific point in a video signal, a filter will apply an aperture function--often sin x/x--centered about the point in question. To produce 32 samples, the filter therefore requires 32 coefficient values calculated in accordance with the aperture function. It will be recognised, however, that the aperture function will generally not remain fixed in position relative to the sample points of the original signal, so that the values of the 32 coefficients are not fixed. Indeed, with relatively close frequencies, the value of each coefficient is likely to change significantly from one sample to the next.
  • the present invention consists, in one aspect, in a process for digitally converting the sample rate of a signal comprising a first digital filtering step of increasing frequency and a second digital filtering step of decreasing frequency; one step utilising a first digital filter of high order serving to change frequency by a factor n which is integral and preferable a power of two, the other step utilising a second digital filter of low order.
  • the order of the first digital filter is significantly in excess of that required in a single filter to meet a desired quality of frequency response and the order of the second digital filter is significantly beneath that required in a single filter to meet the same desired quality of frequency response.
  • n 2
  • Another advantage of filtering to a frequency which is related as a power of two--and, ideally doubled-- is that the aperture function can be made symmetric so as to reduce the number of coefficients. Still better, the aperture function can be arranged such that a number of the coefficients fall to zero.
  • the inventor has further recognised that in the second digital filter, which accommodates sampling frequencies which are not related by an even integer and thus requires variable coefficients, a relatively low number of samples--perhaps between 4 and 8--can be taken without materially degrading the overall frequency response.
  • a filter having a small number of filters or taps can readily be produced with variable coefficients, even at megahertz frequencies.
  • FIG. 1 illustrates schematically a prior art technique for rate conversion
  • FIGS. 2a) and 2b) are diagrams illustrating the principle of rate conversion using digital filters
  • FIG. 3 is a plot illustrating the frequency response imposed by standards
  • FIG. 4 illustrates in schematic form a process according to the present invention
  • FIG. 5 illustrates a modified process according to the invention
  • FIG. 6 is a block diagram of apparatus according to the present invention.
  • FIG. 1 illustrates a prior art technique for rate conversion of a digital signal with an initial sampling frequency of 4 Fsc (four times sub-carrier frequency) to 13.5 MHz.
  • the digital signal is taken to a digital-to-analogue converter 10.
  • the analogue output is taken through a low pass analogue filter 12 and an amplifier 14.
  • the output of the amplifier 14 may optionally be taken directly, or through a further low pass filter 16, to an analogue-to-digital converter 18.
  • the output of the analogue to digital converter 18 is a digital signal sampled at 13.5 MHz.
  • the frequency response of the analogue filter or filters is regulated by industry standards such as CCIR 601 or CCIR 656.
  • industry standards such as CCIR 601 or CCIR 656.
  • the meeting of these standards does not in itself pose any difficulty.
  • the real disadvantage in this approach is the need to return to the analogue environment for the purpose of the rate conversion.
  • Digital filtering can be regarded in one sense as the resampling at a new sample frequency of a notional signal represented by digital values at an old sample frequency.
  • an analogue signal 20 is sampled at a first frequency F to produce digital values represented by ordinates 22, shown in full line. If that digital signal is to be filtered to a new frequency F', fresh digital values will be computed and those digital values can be represented in FIG. a by ordinates 24 shown in dotted line.
  • an interpolation is conducted between neighbouring digital values 22.
  • an aperture function such as that shown in FIG. 2b), centered on the digital value to be computed, is applied to the input digital values.
  • digital filter is conducted in two stages.
  • an input signal at four times the sub-carrier frequency is digitally filtered to double that frequency, in a first digital filter 40.
  • This digital filter is of high order with perhaps 55 taps or samples.
  • the frequency response of such a filter is excellent, as illustrated in the figure.
  • the interpolation process is very much simplified. There are no significant shifts of phase between the aperture function and the digital values from which the interpolation is conducted and the variation in coefficients from one interpolation calculation to the next, is very much reduced. Indeed, if the aperture function is chosen appropriately, the coefficient values will remain constant. As will be appreciated, this considerably simplifies the implementation of the filter.
  • a second digital filtering process is conducted to produce, from the intermediate sample frequency of 8 Fsc, the required sample frequency of, for example 13.5 MHz.
  • This digital filter 42 is of relatively low order having in one example from four to eight taps or samples. Since there is no longer a power of two relationship between the input and output frequencies, there will be significant shifts in phase of the aperture function and the coefficient values represented by that aperture function will vary with time. However, the number of coefficient values (for example four to eight) is very much smaller than the number which would be necessary in a single filter to meet the imposed frequency standard (perhaps 32). It is therefore very much more straightforward to implement the digital filter.
  • the frequency response of the digital filter 42 is poor over the range 0 to 8 Fsc. This is as one would expect for a digital filter having a small number of taps. Since, however, the digital filter 42 is operating at a doubled frequency, the frequency response in the range of interest, can readily be maintained within the CCIR 601 standard.
  • an input frequency at 4 Fsc can be filtered in a digital filter 44 of low order, to an intermediate frequency of, for example, 27 MHz.
  • a second digital filter 46 of high order is then used to filter at the required output sample frequency of 13.5 MHz.
  • An input signal in this case at 13.5 MHz, is taken to a fixed filter 50 which operates at 27 MHz.
  • This filter has high order and in particular is of an order which is higher than that necessary in a single filter to meet a specified frequency response.
  • Such filters are commercially available in integrated devices.
  • the device used is a decimation/interpolation filter, product No. TRW TMC 2242.
  • the output of the fixed filter 50 is taken to a second filter 52 shown in the form of a four coefficient interpolator.
  • the output 54 of the fixed filter 50 is connected with a series of delays 56, each of one pixel.
  • the output 54 and the delayed outputs 58, 60 and 62 are taken through respective multipliers 64 to a summer 66.
  • Each multiplier has a ROM 70 providing a coefficient look-up table.
  • Each ROM 70 receives address information on line 72 which varies with the phase of the interpolation, that is to say with the phase of the aperture function relative to the phase of digital inputs.
  • the output of the summer 66 is taken to a first-in-first-out (FIFO) register which is clocked in at 27 MHz and clocked out at the required output sample frequency, perhaps 4 Fsc.
  • FIFO first-in-first-out
  • digital filter 52 is comparatively straightforward even at video frequency.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • Mathematical Physics (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Analogue/Digital Conversion (AREA)
  • Transmission And Conversion Of Sensor Element Output (AREA)
  • Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
  • Stereo-Broadcasting Methods (AREA)
  • Transmission Systems Not Characterized By The Medium Used For Transmission (AREA)
  • Image Processing (AREA)
  • Picture Signal Circuits (AREA)
  • Measurement Of Radiation (AREA)
US08/211,648 1991-10-10 1992-10-09 Digital sample rate conversion Expired - Lifetime US5602762A (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GB919121504A GB9121504D0 (en) 1991-10-10 1991-10-10 Signal sampling
GB9121504 1991-10-10
PCT/GB1992/001846 WO1993007712A1 (fr) 1991-10-10 1992-10-09 Echantillonnage de signal

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US5602762A true US5602762A (en) 1997-02-11

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US08/211,648 Expired - Lifetime US5602762A (en) 1991-10-10 1992-10-09 Digital sample rate conversion

Country Status (11)

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US (1) US5602762A (fr)
EP (1) EP0623269B1 (fr)
JP (1) JPH07500229A (fr)
AT (1) ATE162355T1 (fr)
AU (1) AU2690892A (fr)
CA (1) CA2120717C (fr)
DE (1) DE69224076T2 (fr)
DK (1) DK0623269T3 (fr)
ES (1) ES2113436T3 (fr)
GB (1) GB9121504D0 (fr)
WO (1) WO1993007712A1 (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5886913A (en) * 1997-05-29 1999-03-23 Alcatel Alsthom Compagnie Generale D'electricite Method of synthesizing a finite impulse response digital filter and filter obtained by this method
US20020097808A1 (en) * 2000-12-01 2002-07-25 Exar Corporation Digitally-controlled line build-out circuit
US20030112904A1 (en) * 2001-11-16 2003-06-19 Fuller Arthur T.G. Time variant filter implementation
US6721427B1 (en) * 1999-06-08 2004-04-13 Zanden Audio System Co., Ltd. Analog filter for digital audio system and audio amplifier for using the same
US20060273938A1 (en) * 2003-03-31 2006-12-07 Van Den Enden Adrianus Wilhelm Up and down sample rate converter

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB9205614D0 (en) * 1992-03-14 1992-04-29 Innovision Ltd Sample rate converter suitable for converting between digital video formats

Citations (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0042392A1 (fr) * 1979-12-21 1981-12-30 NAESS, Erik B Procede et appareil de recuperation de petrole et de gaz a partir d'une rupture sous l'eau
EP0044968A1 (fr) * 1980-07-26 1982-02-03 BASF Aktiengesellschaft Procédé pour la préparation de pipéridines N-alkylées et de pyrrolidines N-alkylées
US4471381A (en) * 1981-03-12 1984-09-11 Victor Company Of Japan, Limited System for converting number of scanning lines
US4472785A (en) * 1980-10-13 1984-09-18 Victor Company Of Japan, Ltd. Sampling frequency converter
US4609941A (en) * 1982-11-30 1986-09-02 British Telecommunications Television signal standards conversion
US4870661A (en) * 1986-09-30 1989-09-26 Kabushiki Kaisha Toshiba Sample rate conversion system having interpolation function
US4903019A (en) * 1987-08-31 1990-02-20 Sanyo Electric Co., Ltd. Sampling frequency converter for converting a lower sampling frequency to a higher sampling frequency and a method therefor
US4949177A (en) * 1988-09-19 1990-08-14 The Grass Valley Group, Inc. Method and apparatus for carrying out a non-linear operation on a digital signal
EP0390531A2 (fr) * 1989-03-30 1990-10-03 Sony Corporation Convertisseur de fréquence d'échantillonnage
EP0443945A1 (fr) * 1990-02-19 1991-08-28 Sony Corporation Convertisseur de la fréquence d'échantillonnage
US5057911A (en) * 1989-10-19 1991-10-15 Matsushita Electric Industrial Co., Ltd. System and method for conversion of digital video signals
US5144450A (en) * 1990-02-27 1992-09-01 Sony Corporation Auto focus frequency conversion filter for ccd imagers having different numbers of pixels
US5335194A (en) * 1992-03-14 1994-08-02 Innovision Limited Sample rate converter

Patent Citations (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0042392A1 (fr) * 1979-12-21 1981-12-30 NAESS, Erik B Procede et appareil de recuperation de petrole et de gaz a partir d'une rupture sous l'eau
EP0044968A1 (fr) * 1980-07-26 1982-02-03 BASF Aktiengesellschaft Procédé pour la préparation de pipéridines N-alkylées et de pyrrolidines N-alkylées
US4472785A (en) * 1980-10-13 1984-09-18 Victor Company Of Japan, Ltd. Sampling frequency converter
US4471381A (en) * 1981-03-12 1984-09-11 Victor Company Of Japan, Limited System for converting number of scanning lines
US4609941A (en) * 1982-11-30 1986-09-02 British Telecommunications Television signal standards conversion
US4870661A (en) * 1986-09-30 1989-09-26 Kabushiki Kaisha Toshiba Sample rate conversion system having interpolation function
US4903019A (en) * 1987-08-31 1990-02-20 Sanyo Electric Co., Ltd. Sampling frequency converter for converting a lower sampling frequency to a higher sampling frequency and a method therefor
US4949177A (en) * 1988-09-19 1990-08-14 The Grass Valley Group, Inc. Method and apparatus for carrying out a non-linear operation on a digital signal
EP0390531A2 (fr) * 1989-03-30 1990-10-03 Sony Corporation Convertisseur de fréquence d'échantillonnage
US5068716A (en) * 1989-03-30 1991-11-26 Sony Corporation Sampling rate converter
US5057911A (en) * 1989-10-19 1991-10-15 Matsushita Electric Industrial Co., Ltd. System and method for conversion of digital video signals
EP0443945A1 (fr) * 1990-02-19 1991-08-28 Sony Corporation Convertisseur de la fréquence d'échantillonnage
US5144450A (en) * 1990-02-27 1992-09-01 Sony Corporation Auto focus frequency conversion filter for ccd imagers having different numbers of pixels
US5335194A (en) * 1992-03-14 1994-08-02 Innovision Limited Sample rate converter

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
P. Pirsch et al., "Changing the Sampling Rate of Video Signals by Rational Factors," Signal Processing II: Theories and Applications,pp. 171-174 (Elsavier Science Publishers B.V., North Holland, 1983).
P. Pirsch et al., Changing the Sampling Rate of Video Signals by Rational Factors, Signal Processing II: Theories and Applications, pp. 171 174 (Elsavier Science Publishers B.V., North Holland, 1983). *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5886913A (en) * 1997-05-29 1999-03-23 Alcatel Alsthom Compagnie Generale D'electricite Method of synthesizing a finite impulse response digital filter and filter obtained by this method
US6721427B1 (en) * 1999-06-08 2004-04-13 Zanden Audio System Co., Ltd. Analog filter for digital audio system and audio amplifier for using the same
US20020097808A1 (en) * 2000-12-01 2002-07-25 Exar Corporation Digitally-controlled line build-out circuit
US20030112904A1 (en) * 2001-11-16 2003-06-19 Fuller Arthur T.G. Time variant filter implementation
US6961395B2 (en) * 2001-11-16 2005-11-01 Nortel Networks Limited Time variant filter implementation
US20060273938A1 (en) * 2003-03-31 2006-12-07 Van Den Enden Adrianus Wilhelm Up and down sample rate converter
US7336208B2 (en) * 2003-03-31 2008-02-26 Nxp B.V. Up and down sample rate converter

Also Published As

Publication number Publication date
ATE162355T1 (de) 1998-01-15
DK0623269T3 (da) 1998-09-14
EP0623269A1 (fr) 1994-11-09
DE69224076D1 (de) 1998-02-19
DE69224076T2 (de) 1998-07-23
AU2690892A (en) 1993-05-03
CA2120717A1 (fr) 1993-04-15
WO1993007712A1 (fr) 1993-04-15
EP0623269B1 (fr) 1998-01-14
CA2120717C (fr) 2003-12-16
ES2113436T3 (es) 1998-05-01
GB9121504D0 (en) 1991-11-27
JPH07500229A (ja) 1995-01-05

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